Heat-developable photosensitive materials which can produce photographic images using a dry heat processing method (referred to herein as "photothermographic materials") are known and described for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and in "Thermally Processed Silver Systems" by D. Morgan and B. Shely, "Imaging Processes and Materials", Neblettes's Eighth edition; Edited by J. M. Sturge, V. Walworth and A. Shepp, p.2 (1969). Such photothermographic materials have a photosensitive medium comprising a reducible silver source, e.g. an organic silver salt; a catalytic amount of photocatalyst, e.g., silver halide, in reactive association with the reducible silver source, and a reducing agent for silver ion, ordinarily dispersed in an (organic) binder matrix. Although stable at ambient temperatures, when heated to higher temperatures, e.g., 80.degree. C., or higher, after imagewise exposure, silver is produced in the exposed regions of the medium via a redox reaction between the reducible silver source and the reducing agent. This redox reaction is accelerated by the catalytic action of the exposure-generated silver catalyst. The silver provides a black image which contrasts with the unexposed areas, resulting in the formation of an image.
In practice, it is essential to include an effective antifoggant in such photothermographic materials, since without an antifoggant, some generation of silver in the unexposed areas takes place upon thermal development, resulting in a poor differential between image and background fog. In addition, one of the problems of photothermographic materials involves their post-processing stability. Since the process is performed without a fixing step, it is desirable to provide a means to enable room light handling of the final image.
Polybrominated organic compounds have been described as both antifoggants and image-stabilisers for photothermographic materials since they can oxidise reduced silver (fog) back to silver bromide under thermal (anti-foggant) and light exposed (image stabiliser) conditions.
Examples of such compounds are described in U.S. Pat. No. 4,546,075, U.S. Pat. No. 4,452,885 (tribromomethylheterocycles as anti-foggants), U.S. Pat. No. 3,874,946 (tribromomethylsulphonylaromatics as image stabilisers) and British Patent Application No. 9221383.4 (tribromomethylketones).
Due to the thermal processing of the photothermographic imaging material, all the compounds used in the construction of the material will be present in the final image sheet. All the materials included in such a material must be acceptable both environmentally and in their possible effect on those persons who might come into contact with the material. The materials must be determined to be non-mutagenic and also it is highly desirable that they do not sensitise the skin of those who come in contact with them. Many polybrominated organic compounds are known to be powerful skin irritants and sensitisers and it is desirable to find compounds having effective anti-foggant and image stability properties for photothermographic materials which exhibit acceptably low sensitisation to human skin.
Our copending British Patent Application No. 9300147.7 discloses compounds of the formula: ##STR2## in which;
R represents a hydrogen atom, an alkyl group, an aryl group or a heterocyclic group, any of which groups may be substituted.
This small class of compounds are effective anti-foggants and image stabilizers in photothermographic materials and exhibit low skin sensitisation. The latter property is particularly surprising since other compounds of similar structure have proved positive in skin sensitisation tests.
The present invention provides a further class of polybromo organic molecules which are suitable for use as image stabilizers and antifoggants in photothermographic materials and exhibit acceptably low sensitisation of human skin and guinea-pigs.